Terrestrial arctic ecosystems store large amounts of carbon (C). With global warming, this C might be released into the atmosphere as CO2 by stimulation of soil microbial degradation. At the same time, CO2 uptake in plants is enhanced, which might, in parts, offset CO2 losses. Yet, the future balance of these two contrasting feedbacks remain uncertain. This study aimed to better understand changes of input and output CO2 fluxes in an arctic tussock tundra ecosystem in response to global warming, with a special focus on the contrast between two sub-ecosystem habitats, the tussocks and the space between tussocks.  An experimental setup was used, where snow fences simulated winter warming by increasing snow depth, and open top chambers simulated summer warming. Daytime ecosystem respiration (ER), reflecting the outward CO2 flux, gross ecosystem production (GEP), reflecting the inward CO2 flux, and net ecosystem exchange (NEE), reflecting the net balance of both, were measured in the summer 2019 in the tussock and the intertussock habitat. In the tussock, both ER and GEP were as twice as high compared to the intertussock and increased with summer warming in a similar magnitude, resulting in an unchanged NEE. Fluxes in the intertussock were not altered with summer warming. Winter warming had no significant effects on ER and GEP in neither of the habitats. However, winter warming increased NEE and green biomass in the intertussock, indicating that in this habitat, plants benefit from warmer winter soil temperatures. Interaction effects of winter and summer warming underline the role of ecological processes outside the summer season, which are to date poorly understood. Contrasting responses of the two sub-ecosystem habitats highlight the challenges in predicting future C balances that are caused by small-scale spatial and temporal heterogeneity of C dynamics.

In the southwest United States, the current prolonged warm drought is similar to the predicted future climate change scenarios for the region. This study aimed to determine patterns in vegetation response to the early 21st century drought across multiple biomes. We hypothesized that different biomes (forests, shrublands, and grasslands) would have different relative sensitivities to both climate drivers (precipitation and temperature) and legacy effects (previous-year's productivity). We tested this hypothesis at eight Ameriflux sites in various Southwest biomes using NASA Moderate-resolution Imaging Spectroradiometer Enhanced Vegetation Index (EVI) from 2001 to 2013. All sites experienced prolonged dry conditions during the study period. The impact of combined precipitation and temperature on Southwest ecosystems at both annual and sub-annual timescales was tested using Standardized Precipitation Evapotranspiration Index (SPEI). All biomes studied had critical sub-annual climate periods during which precipitation and temperature influenced production. In forests, annual peak greenness (EVImax) was best predicted by 9-month SPEI calculated in July (i.e., January-July). In shrublands and grasslands, EVImax was best predicted by SPEI in July through September, with little effect of the previous year's EVImax. Daily gross ecosystem production (GEP) derived from flux tower data yielded further insights into the complex interplay between precipitation and temperature. In forests, GEP was driven by cool-season precipitation and constrained by warm-season maximum temperature. GEP in both shrublands and grasslands was driven by summer precipitation and constrained by high daily summer maximum temperatures. In grasslands, there was a negative relationship between temperature and GEP in July, but no relationship in August and September. Consideration of sub-annual climate conditions and the inclusion of the effect of temperature on the water balance allowed us to generalize the functional responses of vegetation to predicted future climate conditions. We conclude that across biomes, drought conditions during critical sub-annual climate periods could have a strong negative impact on vegetation production in the southwestern United States.

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